examples/ct: convert tests to Go normal form
examples/ct: convert tests to Go normal form

Make tests table driven to reduce copy-pasta.
Avoid t.Fatalf() where possible.
Canonicalize error messages.

gtank/trillian,daviddrysdale/trillian,gtank/trillian,RJPercival/trillian,gdbelvin/trillian,Martin2112/trillian,codingllama/trillian,daviddrysdale/trillian,gdbelvin/trillian,AlCutter/trillian,Martin2112/trillian,codingllama/trillian,google/trillian,google/trillian,RJPercival/trillian,gdbelvin/trillian,AlCutter/trillian,Martin2112/trillian,daviddrysdale/trillian